The 4 mA to 20 mA range is mapped to
represent the normal gas
detection range; current values outside this range can be used to
fault-diagnostic information, as shown in Table 1.
Table 1: Assigning currents outside the 4 mA to
20 mA output range.
PLC Evaluation System
The PLC evaluation system described here integrates all the stages
needed to generate a complete
input/output design. It contains four
fully isolated ADC channels, an ARM7 microprocessor with RS-232
interface, and four fully isolated DAC output channels. The board is
powered by a dc supply. Hardware-configurable input ranges include 0 V
to 5 V, 0 V to 10 V, +/-5 V, +/-10 V, 4 mA to 20 mA, 0 mA to 20 mA,
as well as thermocouple and RTD. Software-programmable output ranges
include 0 V to 5 V, 0 V to 10 V, +/-5 V, +/-10 V, 4 mA to 20 mA, 0 mA
mA, and 0 mA to 24 mA.
Figure 3: Analog input/output module.